The present embodiments relate to medical imaging and, in particular, to surrogate-free four dimensional (4D) computed tomography (CT) reconstruction.
CT is often used for imaging tissues, such as lungs, that do not provide high contrast in MR images. Capturing lung dynamics due to patient respiration during the acquisition is crucial for accurately localizing and treating abnormalities, such as tumors, in the thoracic or abdominal regions. Clinical CT scanners acquire 4D CT data using one of two methods: low pitch helical or ciné acquisition sequences. In helical acquisitions, data are acquired while the table moves through the gantry. In ciné acquisitions, data are acquired in slabs that include only a small part of the body region under investigation. The de facto procedure in the clinic to “solve” this problem is to reconstruct 4D CT data by stitching together consecutive body segments. 4D CT imaging alleviates some of the problems associated with respiratory motion, improving the accuracy of radiation therapy. During a 4D CT scan, a large number of axial images are collected over several breathing cycles. These images are then sorted into a series of 3D volumes corresponding to different breathing phases. Inaccurate sorting often leads to motion and blurring artifacts in the reconstructed volumes.
Most 4D CT image reconstruction methods rely on external respiratory monitoring systems (e.g., bellows-based or real-time position monitoring (RPM) systems) to derive a signal for sorting the large number of images into different respiratory phases. It is assumed that the images within each phase bin were acquired at the same respiratory phase and are combined to form a 3D volume at that phase. This approach has several drawbacks. External respiratory surrogates are often expensive, inconvenient (e.g., requiring calibration) and time consuming. Moreover, mismatches between the external signal and the internal breathing motion may generate volumes suffering from imaging artifacts, especially during irregular breathing patterns.
Several free-breathing techniques have been proposed in the literature to improve 4D CT imaging, attempting to eliminate the need for external surrogates of respiratory motion. These methods may be computationally expensive, require manual intervention, such as to select a reference volume, and/or may fail to remove or sufficiently reduce artifacts.